Voice coil motors (VCM) are commonly used for moving an arm that carries read/write heads over a spinning disk from a rest position on a parking ramp to a desired position, and vice-versa. Commonly, the disk is rotated by a brushless spindle motor such to position the heads over the tracks of a certain sector of the disk from which data is to be read or written
Voice-coil motors are used in a number of applications. They are substantially composed of a winding immersed in a magnetic field generated by a permanent magnet. By forcing a certain current through the winding, the winding receives a displacing force. The displacement of the winding may be controlled with great precision.
In a disk storage device, it is important to control with high precision both the brushless spindle motor and the VCM. Modern fabrication technologies permit fabrication of integrated devices containing in a single package the power switches (bipolar junction or MOS transistors) of the output drivers of the brushless motor, and of the VCM for positioning the heads together with the control circuitries of both motors.
With these technologies allowing fabrication in a single chip through a single process logic CMOS devices, bipolar junction signal transistors, vertical or lateral power MOS or BJT devices, it has been possible to digitally implement many control functions. By using powerful CAD tools it has been possible to implement rather complex functions and controls of extremely high precision and performances in these integrated devices, commonly known as “power combo”.
Co-integrated control architectures based exclusively on digital techniques and circuits and output power stages make these devices adaptable to the user needs, and to different schemes of partitioning of the mass storage support.
The circuit for driving the brushless spindle motor for rotating the disk is generally composed of fully digital circuit blocks Improvements toward a complete digitalization also of the control circuit of the VCM have been achieved, and they are leading to the realization of a control architecture of the VCM motor called “digital power processing voltage mode” or DPPV.
For example, FIGS. 1 and 2 depict respective integrated control systems of a brushless motor and of a VCM for positioning the heads on the rotating disk according to a modern embodiment on a single chip of a power combo device intended for a hard disk device (HDD).
The function of the different circuit blocks and of the main signals of the two control systems, used in various commercial devices of STMicroelectronics, depicted in FIGS. 1 and 2, are illustrated in TABLES 1 and 2:
TABLE 1Self Adapt. T.O.Circuit for rephrasing automatically thecurrent in the spindle with its BEMF inorder to optimize the motor torque.Curr. SignCircuit for determining the direction of theflow of the current in the spindle.ADCAnalog-to-digital converter of the supplyvoltage and of other main voltages.Phase ShiftEnables the rephasing of the current of thespindle and of the relative BEMF upon a userinput or upon a signal coming from theSelf Adapt T.O.Feed-FwdCircuit for feed-forward compensatingdifferences between the supply voltage andthe nominal value.Address generatorIt generates the address of the data storedin “Memory 2 × 3 profile” correspondingto the instantaneous values of the voltageto be given to the phases of the spindlefor generating an appropriate voltagewaveform.Memory 2 by3Look up table that stores the data sequencesprofilethat generate an appropriate voltage waveform(called 2 × 3) to be supplied to the phasesof the spindle during its rotation.Digital multiplierA digital multiplier that allows modulationof the amplitude of each phase voltage ofthe spindle as a function of the value comingfrom the speed controller for allowing thecontrol of the speed.PWM ConverterIn case of three-phase motors, it convertsthree digital data with N bits in threephase modulated digital data (PSM).SPINDLE PSM POWERPower stage for driving (in switched mode)the brushless motor.Spindle RegisterBank of registers for controlling the variousMapfunctions of the spindle.WindowDetermines the duration in electrical degreesof the period between two consecutive zero-crosses of the BEMF in one or more phases ofthe spindle.Mask PWMMasks by an appropriate duration eachswitching of the PSM signals generated bythe block PWM converter.Mask Win.Masks by an appropriate duration in degreeseach time a tristate condition is forced forreading correctly the BEMF.BEMFBack electromotive force induced in thewindings of the brushless motor.FrequencyFrequency multiplier.multiplierZC filterFilter for eliminating spurious switching ofa zero-cross signal ZC generated at eachzero-cross of the BEMF.ZC comp. + ZCIn this block the zero-cross signal ismaskinggenerated and all the maskings forced by thesignals Mask Win and Mask PWM and by thecurrent limiter are enabled.Curr LimCurrent limiterInd S.Block for measuring inductances of the threephases of the used spindle, in the start-upphase, for identifying the position of therotor.Async comm.This signal is completely asynchronous withthe other signals of the system.Embedded startupIntegrated automatic start-up of the spindle.DMUXBlock for selecting the output signals ofthe blocks inductive sense and ZC filter.SpindleBrushless motor that rotates the disk.
TABLE 2F-Fwd ADCAnalog-to-digital converter of the supplyvoltage level.R var. Compensat.Circuit for compensating variations ofresistance of the winding of the VCM inrespect to the nominal value.POR RetractCircuit that displaces the VCM in the restposition when a Power on Reset is asserted.VCM Register MapBank of registers for controlling the variousfunctions of the VCM.IIRInfinite impulse response filter.Pwr Supply Feed-Circuit for feed-forward compensating supplyFwdvoltage shifts from the nominal value.Hi Perform PSMConverts an N bit digital datum in two phase-conv.shift modulated (PSM) signals for driving theVCM.VCM PSM POWERPower stage for driving the VCM.BEMF Compensat.Circuit for compensating the back electromotiveforce induced in the winding of the VCM.Discont. RampCharges/discharges the VCM on/from the parkingLoad.ramp with temporaneous interruption of thecurrent for reading the BEMF.ADCAnalog-to-digital converter.
In the depicted example, the driving circuit of the VCM is realized according to a digital voltage mode. This requires dedicated additional control blocks for compensating unavoidable variations of the three main variables that, in this case, are not controlled, as it happens with a current feedback loop, namely: the back electromotive force induced in the winding, the supply voltage, and the resistance in the winding of the motor.
Obviously, if the control architecture of the VCM is based on a more traditional current control loop, the circuit blocks of FIG. 2, called R var. Compensat., IIR, BEMF Compensat., PWR Supply FF and F-FWD ADC would be substituted in their function by a normal feedback current control circuit (current mode driving).
These integrated devices are known as a power combo, despite the efforts for implementing control functions, safety functions for ensuring the integrity of the power stages and any other possible function in a digital mode. They require the presence of other analog circuits besides the integrated structures of the output power devices.
Constantly improved fabrication technologies for reaching larger and larger scales of integration, especially for digital circuits, have allowed a reduction in the size of devices and in their footprint. These are important factors for the manufacturers of mass storage disk devices,
Despite the new possibilities of greater compactness of the control digital circuitry offered by the new fabrication technologies, these extreme process technologies do not allow the fabrication of power devices and of related analog driving circuits of utmost performance in terms of power consumption. Further reductions of the size must be combined with the need of preserving acceptable performances of the power stages and of the related driving circuits.